The present invention relates to a pneumatic tire with a plurality of tread design elements different from each other in the pitch length and arranged on the tread surface which enables a reduction in the noise (pattern noise) caused by the tread design element accompanying rolling of the tire.
In order to reduce the pattern noise, a proposal has been made in the art on dispersion of the pattern noise in a wide frequency range (a frequency dependent upon the product of the number of revolutions of the tire and that of tread design elements) around the pitch frequency to make the noise inconspicuous. This method is called a variable pitch arrangement. In this method, several kinds of tread design elements (i.e., pitches) different from each other in the pitch length are properly provided in the circumferential direction of the tire so that the time intervals of a pulsatory noise or vibration caused when each tread design element is brought into contact with the ground surface is changed, thereby preventing the noise from concentrating on a particular frequency.
This method is based on frequency modulation theory employed in, for example, radio engineering. However, in this method, no sufficient reduction in the pattern noise can be attained.
The present inventors have made studies with a view to reducing the pattern noise and, as a result, have found that the pulsation of the sound pressure level must not be overlooked as a factor which worsens the impression of the tire noise. Specifically, when the conventional sound level measuring method wherein a sound level is expressed in terms of an average value in a given period of time provides the same sound level, the auditory feeling of the human being frequently finds a difference in the sound level. The present inventors have searched for the cause of the above-described phenomenon and, as a result, have found that this phenomenon is attributable to a difference between the sound pressure level which greatly pulsates in a frequency range as low as about 10 Hz or less and the sound pressure level which does not pulsate in such a frequency range. The pulsation of the sound level, i.e., the pulsation which is one of the main causes of the noise, can be determined by outputting the change in the sound pressure level with time through reproduction at a low speed of the noise recorded at a high speed. For example, the pulsation can be determined according to the testing method for tire noise prescribed in JASO C606-73 wherein a tire is rolled at 50 km/hr on a steel drum having a diameter of 3000 mm (pneumatic pressure, rim size, and load: JATMA standard conditions) to evaluate the pulsation in terms of the degree of variation of OA value (overall value of the noise which has passed through a band-pass filter of 100 to 2000 Hz) caused when the tire is made one turn.
In the theory of the conventional tread design element arrangement with respect to the sound pressure level, it is a common practice to simulate the dispersion on a frequency axis through Fourier expansion of a sine wave train generated at the same time intervals as the order of arrangement of the tread design elements in one turn of the tire, assuming that one sine wave is generated from one tread design element. In particular, various studies and proposals have been made on a theoretical analysis in such an arrangement that the pitches are successively arranged from a short pitch to a long pitch and again to a short pitch, thereby changing the pitch length in a sine wave form (see, e.g., Jidosha Gijutsu, Vol. 28, No. 1, 1974 "Taiya Noizu ni Tsuite", and Japanese Patent Laid-Open No. 115801/1979). In these observations, no discussion is made on the pulsation of the above-described sound pressure level because the observation is made assuming that the amplitude of the vibration generated from each tread design element is constant.
The present inventors have noted that a large circumferential length of the tread design element gives rise to a large level of a vibration generated from the element and have tried a theoretical calculation under the following assumption. That is, Fourier expansion has been made assuming that the vibration generated from each tread design element is a sine wave wherein the amplitude is increased in proportion to the circumferential length of the tread design element. As a result, it has been found that, as is apparent from FIGS. 8(a) and (b) and FIGS. 9(a) and (b), when assuming that sine waves having an equal amplitude are generated from each tread design element according to the conventional calculation method, no amplitude appears in a low frequency range as shown in FIG. 8(b), while when assuming that there occurs a sine wave having an amplitude corresponding to the pitch length of the tread design element, a vibration peak appears in a low frequency range corresponding to a particular periodicity of the tread design element arrangement as shown in FIG. 9(b). In particular, when the arrangement of the tread design elements is regular, the peak in this low-frequency range becomes significant, which enhances the pulsation of the sound pressure level, so that the impression of the noise is worsened.
FIGS. 8(a) and 9(a) are respectively explanatory views of pitch arrangements (tread design element arrangements). Numeral 2 designates a vibration wave form. FIG. 8(b) and FIG. 9(b) are each a graph showing the relationship between the order in the Fourier analysis and the amplitude corresponding to that order. In FIG. 8(a) and FIG. 9(a), the length of pitch A is 31.7 mm, that of pitch B is 27.5 mm and that of pitch C is 24.5 mm, and pitch group E.sub.1 refers to a sequence of C C C C C C, pitch group E.sub.2 of refers to a sequence of B B B B B B B B, pitch group E.sub.3 refers to a sequence of A A A A A A A, pitch group E.sub.4 refers to a sequence of B B B B, pitch group E.sub.5 refers to a sequence of C C C C C C, pitch group E.sub.6 refers to a sequence of B B B, pitch group E.sub.7 refers to a sequence of A A A A A A, pitch group E.sub.8 refers to a sequence of B B B B B B B, pitch group E.sub.9 refers to a sequence of C C C C C C C C C, pitch group E.sub.10 refers to a sequence of B B B B, pitch group E.sub.11 refers to a sequence of A A A A A, and pitch group E.sub.12 refers to a sequence of B B B B. The pitch arrangement shown in FIG. 8(a) is the same as that shown in FIG. 9(a). The term "pitch" used herein is intended to mean the minimum unit of a repeating pattern of constituting a tire tread design comprising a continuous repeating pattern provided in the circumferential direction of the tire. The term "pitch group" is intended to mean a portion wherein a plurality of the identical pitches among the pitches are arranged in sequence. The term "pitch length" is intended to mean the length of the pitch in the circumferential direction of the tire. The term "period" means a pitch arrangement or one of a number of sequentially arranged pitch arrangements which extend circumferentially around a tire tread surface.